1. Field of the Invention
The present invention relates to a method and a circuit for spot-killer control in a cathode-ray tube, more particularly to a method and a circuit for preventing appearance of a zigzag flush of light in a cathode-ray tube at the time of turning it off.
2. Related Art
A television picture tube or cathode-ray tube is equipped with an electron gun, comprising a hot cathode which emits electrons. These are concentrated into a beam which is moved to and fro by a deflecting means and appears as a spot of light on a fluorescent screen. The scanning spot of the electron beam moves to and fro across the screen, line by line. When the cathode-ray tube turns off, it may happen that an intense zigzag flush of light appears in the screen. This is caused by the discharging of remaining static electricity subsequent to the turning-off of the cathode-ray tube. As seen from FIG. 3, when the screen is flooded with electrons, the horizontal scanning happens to stop on the way to the lowermost of the screen by turning the power supply off. Then, the remaining static electricity is allowed to flow to the ground. As a result an intense zigzag flush of light appears in the screen. This lowers the “turning-off” quality, giving unpleasing impression to televiewers.
More specifically, assuming that the horizontal sweeping is made to stop (see FIG. 3(d)) when the vertical sweeping is on the halfway to the lowermost position of the screen (see FIG. 3(e)). The horizontal sweeping signal decays to stop scanning. The vertical sweeping decays to stop later than the horizontal sweeping. As a result, the upper half of the screen shines white, and the lower half turns dark (see FIG. 3(f)). Then, a zigzag flush of light appears in the lower half of the screen.
In the hope of improving the “turning-off” quality JP 2000-115568A, titled “Cathode-ray Tube Display and Method of Turning It Off” proposed that: the time at which the vertical sweeping signal ends is detected subsequent to appearance of the power-off signal; and a ramp signal generator is responsive to the signal representing the so detected termination of the saw-tooth signal for providing an extra high-voltage signal, which falls to zero voltage immediately after its appearance, thus expanding the vertical sweeping beyond the lowermost of the screen, where the spot killer circuit is allowed to discharge the residual storage electricity, thus preventing appearance of the zigzag flush of light in the screen.
JP 9-266541A, titled “Television Receiver” proposed that: the cathode-ray tube is responsive to appearance of the power-off signal to shine white all over the screen for a short while (approximately 20 milliseconds); and then the cathode-ray tube is supplied with the beam current, thereby allowing the residual static electricity to flow to the ground via the CRT drive circuit.
JP 5-191753A, titled “Power-Supply Circuit for a Television Receiver” proposed that: synchronous signals are produced in synchronization with the vertical sweeping; in response to appearance of the power-off signal a decision is made as to whether or not a synchronous signal appears; and in the affirmative case the electric power supply is made to turn off in synchronization with the synchronous signal, and hence with the vertical sweeping, thereby guaranteeing the quality of power-off.
These conventional devices, however, cannot completely prevent the zigzag flush of light from appearing in a cathode-ray tube at the time of turning it off.
One object of the present invention is to provide a method and a circuit for completely preventing appearance of a zigzag flush of light in a cathode-ray tube at the time of turning it off.